In present-day data processing systems, it is the usual practice to employ magnetic memory discs for storing binary bits representing digital data. The memory discs usually comprise a magnetic disc which is scanned by a magnetic transducer head. The magnetic head is capable of inducing flux reversals in the magnetic domains of the disc and, in turn, of reading a pattern of magnetic orientations on the disc, and translating changes in the magnetic orientation into a series of digitally encoded binary bits.
Several types of magnetic head/magnetic memory disc interfaces are used in present-day data processing systems. For example, magnetic tape memories and floppy magnetic disc memories include magnetic heads which are in intimate contact with the magnetic memory. Another type of magnetic memory is known as the Winchester type which uses rigid magnetic discs. The Winchester magnetic disc memory provides maximum reliability and minimum error generation by eliminating physical contact between the magnetic head and the magnetic disc. This is achieved by means of a flying magnetic head which does not actually contact the surface of the magnetic disc.
It is evident that for maximum efficiency it is essential that the actual displacement of the head from the surface of the magnetic disc be kept at a minimum. Present-day systems are available in which the displacement is of the order of 10-14 microns. Accordingly, for satisfactory operation of the Winchester system it is essential that the surface of the magnetic disc be extremely flat and uniform.
The magnetic disc for the Winchester system is currently prepared from a slurry of gamma ferric oxide mixed in a matrix of an organic material capable of forming a thin uniform magnetic film. A rigid disc was used and the magnetic film deposited on the disc was burnished to provide the uniform surface characteristics required in that type of drive.
U.S. Pat. No. 3,634,047, issued to Faulkner, disclosed a method and apparatus for electroplating the magnetic film on the disc substrate so as to provide a magnetic memory disc suitable for use in the Winchester system. However, prior to the electroplating of the main magnetic film, practice is to provide a fine grain paramagnetic film. This is usually achieved by electroless deposition of a film of paramagnetic nickel/phosphorus material. However, prior to the electroplating of the main film, it is necessary for the paramagnetic nickel/phosphorus film, in accordance with the prior art techniques, to be burnished and polished so as to remove some of the nodules that result from the electroless deposition process.
U.S. Pat. No. 3,634,209, issued to Wolf describes a process for producing magnetic memory devices in which the nickel/phosphorus fine grain paramagnetic film is deposited on the substrate by electroplating means, and in which the main magnetic film is then electroplated over the paramagnetic film. However, again, in order to achieve the uniform density required for the Winchester-type of system, the paramagnetic nickel/phosphorus film must be polished and burnished prior to electroplating the main magnetic film.
The requirement for burnishing and polishing in the prior art methods is primarily due to the difficulty of maintaining constant current densities over the entire plating surfaces of the disc during electroplating. In particular, since the thickness of an electrodeposit at any point on a plateable surface is proportional to the time integral of the current density developed during electroplating, the lack of close control over current density in conventional electroplating apparatus has made it very difficult to plate magnetic surfaces capable of high density recording.
Accordingly, the prior art approach to provide a magnetic memory disc capable of high density recording and suitable for use in a Winchester system usually involves the following steps:
(1) An aluminum substrate is prepared by stamping a plate into the proper pre-defined dimensions. Standards have been defined by the American Society for Testing Materials for discs of fourteen inch, eight inch, five and one-quarter inch and 3.1 inch outer diameters.
(2) The substrates are then machined and stress relieved to obtain the finest tolerances possible.
(3) The substrates are then diamond turned and/or polished to an extremely fine finish.
(4) The polished substrates are then subjected to a series of plating operations to place a thin film of fine grain paramagnetic nickel/phosphorus material over the surface of the substrate. This film may be of the order of 0.0002 inches thick. The film may be deposited on the polished substrates either by electroless deposition techniques, or by electroplating as described in the Wolf patent.
(5) The coated substrates are then polished again in an effort to remove some of the nodules that result from the deposition process.
(6) After the polishing operation, the discs are reracked and subjected to an electroplating operation, for example, such as described in the Faulkner patent, so that the main magnetic film may be deposited over the paramagnetic film with the required overall degree of uniformity.
(7) A protective barrier coating may then be formed over the surfaces of the plated disc.
The prior art methods, as described above, are relatively expensive, especially in the requirements of the polishing and burnishing operations. These operations are usually performed manually, and are the leading causes for product failure.
Additional problems occur when the paramagnetic nickel/phosphorus film is deposited by electroless methods due to slight variations in the characteristics of the film over the surface of the disc. These variations result in major changes in the signal response during read/write operations.
An important objective of the present invention is to provide a method and process by which the paramagnetic nickel/phosphorus film may be deposited on the substrate by electroplating techniques, so as to obviate the problems encountered when electroless deposition is used, and by which the paramagnetic film is provided with a high degree of uniformity so as to eliminate any need for the time consuming and expensive manual polishing and burnishing operations.
There is another object of the present invention to provide a method and apparatus for the electrolytic plating of computer hard discs that is suitable for mass production.
It is a further object to the present invention to provide a method and apparatus of electrolytic plating that allows the current densities at the cathodes and anodes to be independently controlled.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.